The ETSI regulations for operation of WLAN's in the 5 GHz band in Europe require the ability to detect primary users, i.e. radar systems. It is likely that similar regulations will be brought into effect for operation in the USA and elsewhere. In order to perform this detection, it is necessary to measure received signal strength during periods when it is known that the local WLAN is not transmitting. Radar signals typically consist of periodic short pulses of radio energy in a fairly narrow beam, which is swept periodically around the radar transmitter. This means that detection of the signal relies on making a measurement when the radar transmitter is directed towards a WLAN receiver and also during an instant when a pulse is being transmitted.
The 802.11h standard provides mechanisms whereby quiet periods can be specified, allowing for such measurements to take place. However, these quiet periods imply reduced throughput, and it may not be possible to meet the regulatory requirements for detecting slow-scanning radars when only using quiet period measurements if such measurements are not to take up an unreasonable proportion of the channel capacity, since the probability that the radar beam is directed towards the WLAN receiver during any measurement period is too low.
The 802.11 WLAN standard defines a number of periods when stations should not be transmitting, the so-called inter-frame spaces (IFS). After each transmitted frame, all stations should wait at least the short inter-frame space (SIFS) duration before transmitting, and in some situations a longer inter-frame space is defined. In principle, it is possible to perform radar detection measurements during such inter-frame spaces and thereby greatly increase the probability of detecting radar signals.